Determination of Picomolar Titanium in Seawater by Isotope Dilution Multicollector Inductively Coupled Plasma Mass Spectrometry after Mg(OH)2 Coprecipitation.

A new isotope dilution inductively coupled plasma mass spectrometry (ICPMS) method is developed to determine picomolar concentrations of titanium (Ti) in seawater. The method applies Mg(OH)2 coprecipitation to concentrate Ti from seawater, and uses a new 49Ti-47Ti isotope dilution to eliminate the need for separating Ti from seawater Ca, resulting in an isobaric interference-free analysis by high-resolution multicollector ICPMS. The method uses a 1.8 mL seawater sample with a detection limit of 1.6 pmol L-1 that is determined mainly by Ti contamination during sample preparation rather than by ICPMS sensitivity, instrumental Ti background, or isobaric interferences. An oceanographically consistent vertical profile of dissolved Ti in the Sargasso Sea near Bermuda is measured with this method.

Spectrophotometric flow injection determination of dissolved titanium in seawater exploiting in-line nitrilotriacetic acid resin preconcentration and a long path length liquid waveguide capillary cell.

A sensitive spectrophotometric method for the determination of dissolved titanium (Ti) in seawater is developed. It involves in-line preconcentration and a long path length liquid waveguide capillary cell (LWCC). Nitrilotriacetic acid (NTA) resin is used to preconcentrate Ti from ∼25 mL seawater sample at pH 1.7, and elution is accomplished with 0.8 mol L-1 hydrochloride acid. The eluted Ti solution is buffered to pH 6.0 with 1.0 mol L-1 ammonium acetate and mixed with 1.5 mmol L-1 Tiron solution. The mixture is then injected into LWCC and measured by spectrophotometry at 420 nm. Before the preconcentration step, the sample is treated with 7 mmol L-1 ascorbic acid to reduce Fe(III) to Fe(II), in order to eliminate the Fe interference. The method is not interfered by Fe(III) and Cu(II) present in seawater samples at concentrations 50-fold higher in relation to Ti, and by Cd(II), Pb(II), Cr(VI), Mn(II), Al(III), Zn(II), and Ni(II) at concentrations 100-fold higher in relation to Ti. It is time efficient (7.5 minutes per sample), sensitive (0.10 nmol L-1 detection limit), precise (1.40% measurement RSD at 1.00 nmol L-1 Ti) and is characterized by a linear range of 0.50-5.00 nmol L-1 Ti. The method was applied to analysis of natural water samples collected from the Jiulongjiang Estuary, Fujian, China.

A catalytic spectrophotometric method for determination of nanomolar manganese in seawater using reverse flow injection analysis and a long path length liquid waveguide capillary cell.

A sensitive and precise method for determination of nanomolar manganese in seawater was developed, using reverse flow injection analysis, a long path length liquid waveguide capillary cell, and spectrophotometric detection. The reaction was based on manganese catalyzed oxidation of leucomalachite green with sodium periodate. Various experimental parameters were investigated and optimized. Foreign trace metal ions of iron, copper, zinc, nickel and aluminum did not cause obvious interference with manganese detection. Low manganese seawater was prepared and used as the blank and standards' matrix, to eliminate the seawater matrix effect. The method detection limit was 0.20nmolL-1, and the quantification range was 0.50-10.00nmolL-1, which should be sensitive enough and suitable for open ocean seawater analysis. The seawater certified reference material NASS-6 was used to test the accuracy, and good agreement was obtained. The proposed method was applied to analyze seawater samples collected at the SEATS station in the South China Sea. The vertical profile of the total dissolvable manganese is reported and discussed.

In-field determination of trace dissolved manganese in estuarine and coastal waters with automatic on-line preconcentration and flame atomic fluorescence spectrometry.

An automatic on-line preconcentration and detection system for analysis of trace dissolved manganese (Mn) in estuarine and coastal waters was established, using preconcentration with IDA chelating resin and detection with flame atomic fluorescence spectrometer (FAFS). The rinse (pre-eluent) solution was optimized, for removing the interference ions while retaining the target element. It was found that the interference ions affected the chelating efficiency of Mn, causing variation of the detection blank and sensitivity. This effect varied when sample volume presented as preconcentration time changed. The influence at preconcentration times of 120 s, 30 s and 10 s were carefully investigated and reported. Ten folds of the foreign trace metals Zn, Cu, Ni, Fe, and Al did not show obvious interference on Mn preconcentration and detection. The method detection limit was 0.9 nmol L-1 (n = 7, preconcentration time 120 s). The linear detection range could be adjusted with designed preconcentration time. In addition to high precision and accuracy, the proposed analytical system had the advantages of high integration, and required normal site preparation, low energy supply and simple auxiliary equipment, which was appropriate for in-field operation. Compared with other common in-field applied molecular spectrometry instruments, the inherent high selectivity and multi-element applicability of FAFS highlighted the superiority and potential of the proposed analytical system. It was successfully applied to in-field vehicle-board determination of dissolved Mn in coastal waters around Xiamen, Fujian, China, and it was also used to analyze natural water samples collected from the Jiulongjiang Estuary, Fujian, China.

Sequential determination of multi-nutrient elements in natural water samples with a reverse flow injection system.

An integrated system was developed for automatic and sequential determination of NO2-, NO3-, PO43-, Fe2+, Fe3+ and Mn2+ in natural waters based on reverse flow injection analysis combined with spectrophotometric detection. The system operation was controlled by a single chip microcomputer and laboratory-programmed software written in LabVIEW. The experimental parameters for each nutrient element analysis were optimized based on a univariate experimental design, and interferences from common ions were evaluated. The upper limits of the linear range (along with detection limit, µmolL-1) of the proposed method was 20 (0.03), 200 (0.7), 12 (0.3), 5 (0.03), 5 (0.03), 9 (0.2) µmolL-1, for NO2-, NO3-, PO43-, Fe2+, Fe3+ and Mn2+, respectively. The relative standard deviations were below 5% (n=9-13) and the recoveries varied from 88.0±1.0% to 104.5±1.0% for spiked water samples. The sample throughput was about 20h-1. This system has been successfully applied for the determination of multi-nutrient elements in different kinds of water samples and showed good agreement with reference methods (slope 1.0260±0.0043, R2=0.9991, n=50).

A flow-batch manipulated Ag NPs based SPR sensor for colorimetric detection of copper ions (Cu2+) in water samples.

Using flow-batch analysis, a highly sensitive and selective method for automatic colorimetric detection of copper ions (Cu2+) was produced on the basis of the surface plasma resonance (SPR) of silver nanoparticles (Ag NPs). The Ag NPs were catalytically etched by thiosulfate in the presence of Cu(NH3)42+, resulting in a color change of the solution induced by the absorbance decrease at 401nm of the SPR peak of Ag NPs. The proposed method showed high selectivity for Cu2+ over various metallic ions, including Fe3+, Mn2+, Co2+, Ni2+, Zn2+, Pb2+, Ba2+, Cd2+, Bi3+, Sb2+, As3+, Hg2+, Cr3+ and K+. The linear range was 0.5-35µg/L with a coefficient of 0.9954. The limit of detection was as low as 0.24µg/L. The relative standard deviation (RSD, n=7) for the determination of Cu2+ spiked samples at concentrations of 10µg/L was 1.21% and for 25µg/L was 1.03%. The proposed method was successfully applied to analyze Cu2+ in lake water, tap water, rainwater and bottled water samples, as well as leaf samples for food packaging. The results were in good agreement with those obtained by graphite furnace atomic absorption spectrometry, the classical technique.

Redox speciation analysis of dissolved iron in estuarine and coastal waters with on-line solid phase extraction and graphite furnace atomic absorption spectrometry detection.

An automatic on-line solid phase extraction (SPE) system employing the flow injection (FI) technique directly coupled to a graphite furnace atomic absorption spectrometer (GFAAS) was established for speciation and determination of dissolved iron in estuarine and coastal waters. Fe(II) was mixed with ferrozine solution in a sample stream to form the Fe(II)-ferrozine complex which was extracted onto a C18 SPE cartridge, eluted with eluent and detected with GFAAS. In a parallel flow channel, Fe(III) was reduced to Fe(II) with ascorbic acid and then detected in the same way as Fe(II). The home-made interface between FI-SPE and GFAAS efficiently realized the sample introduction to the furnace in a semi-automated way. Parameters of the FI-SPE system and graphite furnace program were optimized based on a univariate experimental design and an orthogonal array design. The salinity effect on the method sensitivity was investigated. The proposed method provided a detection limit of 1.38 nmol L(-1) for Fe(II) and 1.87 nmol L(-1) for Fe(II+III). With variation of the sample loading volume, a broadened determination range of 2.5-200 nmol L(-1) iron could be obtained. The proposed method was successfully applied to analyze iron species in samples collected from the Jiulongjiang Estuary, Fujian, China. With the 2-cartridge FI-SPE system developed, on-line simultaneous determination of Fe species with GFAAS was achieved for the first time.

Real-time redox speciation of iron in estuarine and coastal surface waters.

An automated, shipboard-use system was developed for real-time speciation of iron in coastal surface waters. It comprised a towed Fish underway sampler and a modified reverse flow injection analysis system with a liquid waveguide capillary flow cell-spectrophotometric detection device. The detection was based on the reaction between ferrozine and Fe(II). The detection limits of 0.3 and 0.7 nM were achieved for Fe(II) and Fe(II+III), together with their respective dynamic linear ranges of 0.5-250 and 0.9-250 nM. The system was successfully deployed and run consecutively for about 1 week during a cruise in August 2009 to the East China Sea off the Changjiang Estuary. The distribution of operationally defined field dissolvable Fe(II) and Fe(II+III) (expressed as Fea(II) and Fea(II+III)) in these areas was obtained, which showed that both Fea(II) and Fea(II+III) concentrations decreased with salinity when there were relatively high Fea(II) concentrations (up to about 120 nM) near shore. A distinct distribution of Fea(II) to Fea(II+III) ratios was also revealed, with a ratio of 0.58 in the water off Changjiang Estuary and 0.19 in the open ocean.

Simultaneous determination of nanomolar nitrite and nitrate in seawater using reverse flow injection analysis coupled with a long path length liquid waveguide capillary cell.

A reverse flow injection analysis (rFIA) method coupled with 1m liquid waveguide capillary cell and spectrophotometric detection for simultaneous determination of nanomolar nitrite and nitrate in seawater was developed. The design of two analytical channels sharing the same detection system in the proposed method allowed the analysis of both nitrite and nitrate with single sample injection. Different strategies of reagent injection were investigated to obtain a higher sensitivity and a better peak shape. A dual-wavelength detection mode was chosen to eliminate the light source shifting and sample matrix interference. Experimental parameters were optimized based on a univariate experimental design and the matrix effect from seawater was preliminarily investigated. The proposed method had high sensitivity with detection limit of 0.6 nmol L(-1) for both nitrite and nitrate. The linearity was 2-500 nmol L(-1) for both analytes, and the upper limit could be extended by choosing a lower sensitivity detection wavelength. The analytical results of 26 surface seawater samples obtained with the proposed method showed good agreement with those using a reference method operated using an automated segmented flow analyzer. The proposed method could greatly minimize the trouble introduced by bubbles in the segmented flow analyzer. It also had the advantages of high precision and high sample throughput (nitrite and nitrate detected in triplicate; 5 h(-1)). Compared to normal flow injection analysis, the rFIA method is superior due to its lower reagent consumption, less dispersion of sample, as well as higher sensitivity.

A sensitive flow-batch system for on board determination of ultra-trace ammonium in seawater: Method development and shipboard application.

Combining fluorescence detection with flow analysis and solid phase extraction (SPE), a highly sensitive and automatic flow system for measurement of ultra-trace ammonium in open ocean water was established. Determination was based on fluorescence detection of a typical product of o-phthaldialdehyde and ammonium. In this study, the fluorescence reaction product could be efficiently extracted onto an SPE cartridge (HLB, hydrophilic-lipophilic balance). The extracted fluorescence compounds were rapidly eluted with ethanol and directed into a flow cell for fluorescence detection. Compared with the common used fluorescence method, the proposed one offered the benefits of improved sensitivity, reduced reagent consumption, negligible salinity effect and lower cost. Experimental parameters were optimized using a univariate experimental design. Calibration curves, ranging from 1.67 to 300nM, were obtained with different reaction times. The recoveries were between 89.5 and 96.5%, and the detection limits in land-based and shipboard laboratories were 0.7 and 1.2nM, respectively. The relative standard deviation was 3.5% (n=5) for an aged seawater sample spiked with 20nM ammonium. Compared with the analytical results obtained using the indophenol blue method coupled to a long-path liquid waveguide capillary cell, the proposed method showed good agreement. The method had been applied on board during a South China Sea cruise in August 2012. A vertical profile of ammonium in the South East Asia Time-Series (SEATS, 18° N, 116° E) station was produced. The distribution of ammonium in the surface seawater of the Qiongdong upwelling in South China Sea is also presented.

An automatic gas-phase molecular absorption spectrometric system using a UV-LED photodiode based detector for determination of nitrite and total nitrate.

An automatic gas-phase molecular absorption spectrometric (GPMAS) system was developed and applied to determine nitrite and total nitrate in water samples. The GPMAS system was coupled with a UV-light emitting diode photodiode (UV-LED-PD) based photometric detector, including a 255 nm UV-LED as the light source, a polyvinyl chloride (PVC) tube of 14 cm as the gas flow cell, and an integrated photodiode amplifier to measure the transmitted light intensity. The UV-LED-PD detector was compact, robust, simple and of low heat production, comparing with detectors used in other GPMAS works. For nitrite measurement, citric acid was used to acidify the sample, and ethanol to catalyze the quantitative formation of NO(2). The produced NO(2) was purged with air flow into the UV-LED-PD detector, and the gaseous absorbance value was measured. The total nitrate could be determined after being reduced to nitrite with a cadmium column. Limits of detection for nitrite and nitrate were 7 µmol/L and 12 µmol/L, respectively; and linear ranges of 0.021-5 mmol/L for nitrite and 0.036-4 mmol/L for nitrate were obtained. Related standard deviations were 1.81% and 1.08% for nitrite and nitrate, respectively, both at 2 mmol/L. The proposed method has been applied to determine nitrite and total nitrate in some environmental water samples.